So I started a ~150ml yeast starter last friday from a small canning jar (those half pint ones) of saved german ale yeast from december. I can only get LME from my homebrew shop so I mixed up a gallon or so and canned or froze most of it. I didn't pay a lot of attention when I took the gravity reading, but it was between 1.047-1.042. Saturday morning I stepped it up to ~750ml and hooked it up to an aquarium pump (through a filter) to oxygenate and agitate the culture.

It appeared that sunday night I had some krausening going. This morning I cold crashed the starter because I knew I was brewing tonight (took it off the air supply and put it down at ~38 degrees). Brewing went pretty smooth, as I was cooling the wort I pulled the starter out to warm up (It was in a 2 liter bottle with the cap on tight). The bottle got pressurized pretty quickly and I opened it twice before everything was finished to off gas (and got that pfftst sound both times) and there as a healthy layer of trub with a thick white layer.

I poured some of the top of the starter in my test glass and it foamed up like crazy... so I figured because of the increase in trub, and the krausening and the off-gassing when it was warming up that I had a healthy culture, and I pitched the rest of it in my wort (I was hoping to just pitch the trub...but 1007 is a poor floccing strain so I pitched the whole thing). The punchline is that the final gravity of the starter was 1.042 (about where it started)!

It also smelled somewhat like yeast, but was much stronger of that oxidized cardboard smell.

Sorry for the long story... question is this. Would you trust there are viable yeast? Should I wait 72 hours? Or should I pitch my backup Notty just to be safe? could the off gassing have just been from warming up and releasing the dissolved gasses?

what are you brewing? Nottingham is always a good choice in my book as a general purpose yeast. As for your jar situation...whenever i think i have a contaminated starter for whatever reason i dont use it. I would rather be out 7 bucks for new yeast than 30-40 for all the grain and hops of a batch i had to toss. Did you remeber to sanitize the jar? If the gravity is the same then obviously either you messed up with the gravity number or you have a contaminated culture.... If it comes down to it, toss in the nottingham

I'm guessing I screwed up the initial gravity number...because 10 hours later the lid is bubbled up...so something is in there eating my wort.

I pitched it assuming it was okay...but I have yet to have a contamination issue so I don't know about differences in smells from other yeast. (Also I was resmelling the jar after my first post and it was maybe less cardboard and more fruity....either way its a flavor I've had in a few commercial beers that I can't stand).

How often do you get a contaminated starter and how are they identified (smell)?

Even if there was contamination...wouldn't the gravity drop more than a few points?

Yes, if it was contaminated the gravity would typically drop substantially. Sometimes a lot more than what the yeast could do on their own.

Contaminating a starter is not something that happens much, at least in my brewery. I think I've only had it happen once in the past eight years that I am certain of and that was due to keeping it around longer than I had planned due to scheduling problems.

The smell test is about all you can do quickly and easily. Don't expect the starter to smell wonderfully. There's usually no hops it them and they are often fermented at relatively high temperatures. What you are trying to detect is any sourness in the wort. If you're still uncertain after the smell test, do a taste test. That should nail it down solidly. Again, it won't taste anything like a finished beer cooled and carbonated, but you should be able to detect a ruined starter without too much trouble.

When seriously in doubt, dump it and pitch a couple of backup dry yeast packets as others have suggested. Dry yeast has recently come into its own. I want to try a lager using dry yeast to see if it will perform as advertised.

Do yourself a favor. Get a two liter erlenmeyer flask and build yourself a stir plate. Making starters should be a slam dunk operation. The procedure becomes very routine and reliable. You have more important things to focus on as brew day approaches such as the recipe and your brewing procedures. You are violating the RDWHHB philosophy. Brewing should not be a stressful experience in any phase.